4 Answers
4

One possibility is that there is a background function that attempts to find suspicious voting patterns, and undoes them, which can result in a dramatic change in reputation for some users. In some cases this is because a user has setup a shell account to upvote everything they do. However, sometimes it is simply because someone decided to follow everything you do and upvote it, which at some point the algorithm may detect and undo the upvotes.

Could be any number of reasons. Like @yshuditelu pointed out, there's a background function that checks for suspicious activity; you might have set it off for some reason. As to specifically why, I'm not sure.
–
DonutSep 23 '09 at 15:15

Actually, looking at your history of questions, I notice you had a bounty on some of them, and accepted answers on them two days ago. You also edited someone else's question, and answered it, gaining a bounty from that. It's all that activity in a short time period from someone fairly low on reputation to begin with that probably looks suspicious to the automated checkers.